It's a factor of how long vaccines have been in the trial subject's arms, which is only 3-4 months as of now. The subjects are still being followed so the duration of effectiveness will continue to be measured. I understand the big question is whether it lasts for only several months so that annual boosters will be needed. There's some speculation that it may be several years, once again only time will tell.You would think trials would have given a better picture of effectiveness
The bigger hindsight issue is whether just one dose would have worked. Looks like it might but the trials weren't set up for that. If they had and it worked then double the numbers could be getting shots right now.